target/loongarch: Implement vavg/vavgr

This patch includes:
- VAVG.{B/H/W/D}[U];
- VAVGR.{B/H/W/D}[U].

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20230504122810.4094787-11-gaosong@loongson.cn>
5 files changed